During lengthy Twitter chat, Tesla boss reveals his self-driving trucks are making progress.

During a coffee-fuelled two-hour Twitter exchange with fans and journalists, Elon Musk revealed Tesla is making progress on the development of self-driving trucks, but the Model 3, its cheapest car yet, remains an "overwhelming priority" for the company.

Musk also revealed a major software update to Tesla's latest self-driving system is "almost there", but the company is "being very careful with the rollout."

Called Tesla Semi, the plan to build electric, autonomous trucks was a part of part two of Musk's 'top secret master plan' for Tesla's future. Published in July 2016, the plan outlines how heavy-duty trucks and urban transport vehicles are "in the early stages of development...and should be ready for unveiling next year [2017]."

Musk said Tesla's truck division will "deliver a substantial reduction in the cost of cargo transport, while increasing safety and making it really fun to operate."

But the trucks are not Musk's immediate priority. This remains the Model 3, which was revealed in April 2016 and is due to go on sale before the end of 2017. It is set to cost around £35,000, making it the cheapest Tesla to date by some £30,000.

The car is expected to have a range of over 215 miles and will act as an all-electric alternative to cars like the BMW 3-Series and Audi A4. With over 400,000 pre-orders already placed for the Model 3, Tesla is under pressure to complete its so-called Gigafactory One and produce the cars in high enough quantity to meet demand. For context, Tesla delivered around 76,000 cars in 2016.

Of course, this being Musk, the colonisation of Mars was also on the agenda for the impromptu Twitter chat. The planet is "critical for the long-term survival of humanity and life on Earth as we know it," Musk said.

The Tesla and SpaceX boss also spent some time pouring cold water on concerns over his position on President Trump's economic advisory council. "Good grief, I'm not 'working' for Trump," Musk said in reply to a tweet suggesting he was losing Tesla sales as a result of advising the US President. "I'm trying to provide advice that helps take things in a more positive direction," he added.

Speaking to Fred Lambert, a journalist from car and clean energy blog Electrek, and who steering much of the Twitter conversation, Musk said in response to his political tweets, he hoped "to get back to regularly scheduled programming soon...".